1. Field of the Invention
The present invention relates to a duplex printing system and a duplex printing method for forming an image etc. on double sides of a web, and particularly to a duplex printing system having an alignment control device for accurately aligning images of double sides.
2. Description of the Related Art
As a printing system for forming an image on double sides of a web typified by long continuous band-shaped paper, a printing system constructed so that two printers are arranged in series and printing is done on the first side (front) of the web by the printer of a forward stage and the front and back of the web delivered from the printer of the forward stage are reversed by a reversing device and then the web is fed into the printer of a backward stage and printing is done on the second side (back) of the web by the printer of the backward stage has been proposed and commercialized.
Recently, as the web used in such a printing system, a printing system capable of coping with a web without feed holes in addition to continuous paper of a form having feed holes in both edges has become widespread. In the printing system as described above, in the case of a printer of a type in which at least a printer arranged in a forward stage forms an image using an electrophotographic method, due to thermal action in a thermal fixing process in which an image (toner image) transferred on a web is melted and fixed in the web, the web fed into a printer of a backward stage thermally shrinks more than the original state and a page length at the time of printing of the front is different from a page length at the time of printing of the back, so that a phenomenon in which an image position of the front side formed on the web does not match with an image position of the back side occurs. In order to cope with such a phenomenon, a printing system using a control method in which an alignment mark is formed in a defined position of the web, for example, the head of a print page in the first printer and timing or an interval of the alignment marks is measured by the second printer and the image position of the front side is matched with the image position of the back side by changing a transport speed of the web from this measured result has been implemented. (Please see JP-A-7-237336 and JP-A-2002-187660.)